1. Field of the Invention
The present invention is directed to a device and method for reducing transmitter induced cross modulation in a receiver, such as in code division multiple access (CDMA) telephones or dual mode telephones operating in the cellular and Personal Communication System (PCS) bands of 900 MHz and 1.9 GHz, respectively.
2. Discussion of the Prior Art
A major problem faced in reducing the size of Code Division Multiple Access (CDMA) mobile cellular telephone handsets is the high isolation requirement between the transmitter and the receiver sections, as they are often both operational simultaneously. The transmitter and the receiver connect to the antenna through a duplexer. Typically, duplexers have a very large size in order to provide sufficient isolation between the transmitter and receiver, e.g., in excess of 60 dB.
With the reduction in size of handsets, especially in dual-band CDMA/AMPS designs, where AMPS is Advanced American Mobile Service (AMPS), the duplexers are also becoming smaller at the expense of reduced isolation between the transmitter and receiver ports. Typically, Personal Communication System (PCS) band duplexers have about 45 dB isolation, while the cellular band duplexers have 45-50 dB isolation. This reduced isolation can cause significant cross modulation due to 3rd order nonlinearity of a low noise amplifier (LNA) of the receiver section.
The cross modulation noise results from the modulated transmitter leakage signal of the handset itself from its transmitter section into its receiver section being combined with a large single tone jammer signal in the receive frequency band. The jammer signals result from adjacent channel signals, such as narrow band modulated signals of adjacent base stations, for example.
In particular, due to the third order non-linearity of the LNA, the single tone jammer gets amplitude modulated from the amplitude variations of the transmitter leakage signal. The transmitter leakage signal produces a time varying desensitization of the LNA. Desensitization is the change in the level of a small signal, in the presence of a large signal, at the LNA output.
To reduce the cross modulation power, a highly linear LNA with a large third order intercept point (IP3) requirement may be needed. However, such a highly linear LNA with a large IP3 consumes a large current and thus quickly drains the battery of mobile phones/transceivers. Accordingly, there is a need to reduce the cross modulation power without the need to use highly linear LNA with a large IP3 requirement, and thus minimize current or power consumption of a transceiver.
The object of the present invention is to provide a controlled receive device and a control method which reduces the problems of conventional receive devices.
Another object of the present invention is to provide a receive or a transceiver device which has reduced cross modulation noise without the need for an inherent highly linear LNA having a large IP3 requirement.
The present invention accomplishes the above and other objects by providing a transceiver having a baseband unit which outputs baseband signals and a modulator which modulates the baseband signals to output a modulated signal. A transmitter amplifier amplifies the modulated signal and outputs a transmit signal to a duplexer. The transceiver further includes a receive amplifier which receives a receive signal from the duplexer, where the receive amplifier has a gain which is modulated by an envelope signal. The envelope signal is proportional to the transmit signal, such as being proportional to the power or square root of the power of the transmit signal.
The envelope signal is provided by the baseband unit. In a preferred embodiment, the envelope signal is provided by an envelope detector which receives the output of the receive amplifier. The envelope signal may also be provided by a rectifier which rectifies the output of the receive amplifier.
In another embodiment, a method for reducing transmitter induced cross modulation in a receiver is provided, which includes the steps of:
forming baseband signals by the baseband unit;
modulating the baseband signals to form a modulated signal;
amplifying the modulated signal to provide a transmit signal to a duplexer;
receiving a receive signal by a receive amplifier from the duplexer to form an amplified receive signal; and
modulating the gain of the receive amplifier using an envelope signal which is proportional to the transmit signal, such as being proportional to the power or square root of the power of the transmit signal.